1. Field of the Invention
The invention relates to flyback switch-mode supplies and more particularly to such power supplies provided with a power factor correction system.
2. Discussion of the Related Art
Switch-mode power supplies with a power factor correction are known. An example is disclosed in the publication of the RCA laboratories, MRZ-339, March 1992, entitled "Single Converter High Power Factor SWITCH-MODE POWER SUPPLY" by M. Megeid.
The attached FIG. 1 reproduces the main elements of FIG. 2A of this document.
Terminals A and B of the mains network are connected to a full wave rectifying bridge BR, having positive and negative output terminals X and Y connected to a primary winding W1 in series with a switch S1 of a switch-mode flyback supply source. This switch-mode supply source comprises a secondary winding W2 coupled to winding W1 and to a rectifier so that no current may flow in winding W2 when winding W1 is conductive and that a current is transferred only when the conduction in winding W1 is terminated. The secondary circuit of this switch-mode supply source conventionally comprises a diode D in series with a storing capacitor C. A load L is parallel-connected with capacitor C.
Between terminals X and Y is arranged a storage capacitor C1. The cited document provides a specific arrangement to improve the power factor, that is for the mains network to provide a current during a time that is longer than the time necessary for charging the capacitor. This arrangement comprises, between terminals X and Y, the serial connection of capacitor C1, a diode D1 and a switch S2. Diode D1 is biased for inhibiting the charging of capacitor C1 by the rectified voltage present between terminals X and Y. The charging of capacitor C1 is due to the connection of this capacitor C1 across an auxiliary winding W3 in series with diode D2. Diode D2 is biased so that capacitor C1 may be charged. According to a feature of this circuit, winding W3 is coupled with winding W1 in the flyback mode, that is, it operates with winding W1, not as a voltage transformer, but for conducting an alternating current.
An advantage of this circuit, as explained in the above article, is that any desired value of the voltage across capacitor C1 can be selected.
However, this circuit has two major drawbacks.
A first drawback is that, due to the flyback connection of winding W3 with respect to winding W1, like winding W2 with respect to winding W1, the voltage across capacitor C1 is linked to the regulated voltage determined by switch S1. Therefore, this voltage does not follow the mains voltage.
A second drawback is that, due to the flyback connection of winding W3 with respect to winding W1, it is necessary to provide a switch S2 connected as shown. This switch S2 has the drawbacks of being costly and of needing a control circuit.
Another drawback of this circuit is that the power available on winding W3 results from the energy provided by winding W1. Therefore, while switch S1 is closed, the current in the switch is more important than the current that is normally necessary for the switch-mode supply, whereby it is necessary to oversize switch S1 and therefore increase its cost.